The present invention relates to the electric supply of light-emitting loads, in particular light-emitting diode (LED) lamps. More specifically, the present invention is concerned with electric circuits and methods required for remote monitoring of LED lamps.
Light-emitting diode (LED) lamps are becoming more and more popular in automotive traffic lights, railway signal lights and other applications. Their lower power consumption is an attractive feature, but the main reason for their popularity is their long life (100 000 hours) compared to standard incandescent lamps (5 000 hours). Manifestly, these features allow important reduction in maintenance costs.
In certain applications, such as railway signal lights, these lamps may be used, as those skilled in the art would know, for main line signalling and/or grade crossing signalling. Grade crossing signals are usually situated in populated areas such as road intersections. Remote monitoring of the LED lamps in grade crossing signals is therefore not necessary. Main line signals, on the other hand, can be installed in remote areas, which are not easily accessible. Remote monitoring for checking the integrity of the lamps signals is therefore common practice.
For lamps equipped with standard incandescent bulb, electrical integrity can be easily verified. If the filament of the incandescent bulb is in normal condition, current flows through the bulb according to Ohm""s law (I=V/R). Otherwise, if the filament is open, no current flows through the bulb and it should be replaced.
For LED lamps, however, LED current is controlled by a power supply. Current characteristics are therefore not identical in a LED lamp and in an incandescent lamp. In a LED lamp, alternative current (ac) line voltage is rectified and then converted to a suitable level by a dc-dc (direct current) converter, which also regulates LED current. In case of LED failure, or failure of any other electrical component in the LED lamp, it is possible for the power supply to continue drawing current at or near the nominal current value, even if the LED""s are not emitting any light. Remote monitoring systems could therefore see the LED lamp as functioning correctly when in reality it is not. This situation is not acceptable since it can lead to very hazardous train operations and cause major accidents.
Another problem, related to LED lamps and their power supplies and controllers, is caused by electric components which retain residual voltage differentials after power is removed from the LED lamp. The resulting characteristic is that a LED lamp will effectively light up when the power applied to it reaches a first high level while it will be turned off only when the power reaches a second lower level. The resulting problem is that if a certain power is induced by, for example, other nearby cables, the LED lamp could remain on while in fact it should be off. This could also lead to dangerous situations.
These particularities of LED lamps limit their widespread use in situations where they need to be remotely monitored such as in railway main line signalling applications.
An object of the present invention is therefore to allow LED lamps to become compatible with remote detection systems designed for monitoring of incandescent lamps.
Another object of the invention is to provide LED lamp circuitry which will emulate an incandescent lamp""s behaviour upon remote monitoring of the LED lamp.
Yet another object of the invention is to provide a control circuit for enabling/disabling the power supply to LED lamps in relation to the level of the line voltage.
More specifically, in accordance with the present invention, there is provided a fuse blow-out circuit for establishing a short circuit between first and second voltage and current supply lines to blow out a protection fuse through which a current supplied to a light-emitting load by the first and second lines flows, this fuse blow-out circuit comprises:
a timer means responsive to the voltage across the first and second lines for producing a time-representative signal after a certain period of time;
means connected to the timer means for preventing production of the time-representative signal in response to the current supplied to the light-emitting load; and
means for establishing a current path between the first and second lines in response to the time-representative signal.
Accordingly, when no current is supplied to the light-emitting load, the current path is established and provides the short circuit between the first and second lines that will blow out the protection fuse and emulate an open circuit of a defective incandescent lamp.
Also in accordance with the present invention, there is provided a fuse blow-out circuit for establishing a short circuit between first and second voltage and current supply lines to blow out a protection fuse through which a current supplied to a light-emitting load by the first and second lines flows. This fuse blow-out circuit comprises:
a resistor and a capacitor connected in series between the first and second lines, this resistor having a given resistance value, and this capacitor having a given capacitance value and a capacitor charge period dependent on the given resistance value and the given capacitance value;
a trigger circuit connected in parallel with the capacitor, and comprising a first controllable switch member closed in response to the current supplied to the light-emitting load to discharge the capacitor; and
a second controllable switch member defining a current path between the first and second lines and closed in response to a given voltage amplitude across the capacitor.
Therefore, in the absence of current supplied to the light-emitting load for a duration equivalent to the capacitor charge period, the given voltage amplitude across the capacitor is reached to thereby close the second switch member, establish the current path and provide the short circuit between the first and second lines that will blow out the protection fuse and emulate an open circuit of a defective incandescent lamp.
Further in accordance with the present invention, there is provided a power supply unit responsive to alternating voltage and current from an ac source for supplying a dc voltage and current to a light-emitting load, comprising:
a rectifier unit rectifying the alternating voltage and current from the ac source and supplying the rectified voltage and current to first and second voltage and current supply lines;
a protection fuse through which the alternating current from the ac source is supplied to the rectifier unit;
a converter of the rectified voltage and current into the dc voltage and current supplied to the light-emitting load;
a fuse blow-out circuit as described above, for establishing a short circuit between the first and second voltage and current supply lines to blow out the protection fuse; and
a controller of the converter in response to the rectified voltage on the first and second lines.
The present invention also relates to a cold filament detection circuit connected between first and second lines through which a voltage and current supply source supplies voltage and current to a light-emitting load, the voltage and current supply source having a set up time during which no current is supplied to the light-emitting load. This cold filament detection circuit comprises:
a resistor;
means for connecting the resistor between the first and second lines in response to the voltage on the first and second lines to thereby establish through this resistor a current path between the first and second lines; and
means for disconnecting the resistor from between the first and second lines in response to the current supplied to the light-emitting load.
Accordingly, during the set up time no current is supplied to the light-emitting load and the current path is established through the resistor to emulate the impedance of an incandescent lamp, and when current is supplied to the light-emitting load, the resistor is disconnected from between the first and second lines.
The present invention further relates to a cold filament detection circuit connected between first and second lines through which a voltage and current supply source supplies voltage and current to a light-emitting load, the voltage and current supply source having a set up time during which no current is supplied to the light-emitting load. The cold filament detection circuit comprises:
a resistor;
a controllable switch member: connected in series with the resistor between the first and second lines; responsive to the voltage on the first and second lines; and having a current-conductive junction established in response to the voltage on the first and second lines to thereby establish through the resistor a current path between the first and second lines; and
a switch control unit responsive to the current supplied to the light-emitting load, connected to the first controllable switch member, and having a switch-disabling circuit which prevents the current-conductive junction to establish as long as current is supplied to the light-emitting load.
In operation, during the set up time no current is supplied to the light-emitting load and the current path is established through the resistor to emulate the impedance of an incandescent lamp, and when current is supplied to the light-emitting load, the switch-disabling circuit prevents the current-conductive junction to establish whereby the resistor is disconnected from between the first and second lines.
The present invention still further relates to a voltage and current supply source responsive to alternating voltage and current from an ac source for supplying dc voltage and current to a light-emitting load, comprising:
a rectifier unit rectifying the alternating voltage and current from the ac source and supplying the rectified voltage and current to first and second voltage and current supply lines;
a converter of the rectified voltage and current into the dc voltage and current supplied to the light-emitting load;
a cold filament detection circuit as defined above, connected between the first and second lines through which the voltage and current supply source supplies voltage and current to the light-emitting load; and
a controller of the converter in response to the rectified voltage on the first and second lines.
The present invention is also concerned with a voltage control circuit for controlling the amplitude of a voltage signal on a control terminal of a power controller unit itself controlling a voltage and current supply source which supplies a current to a light-emitting load through first and second voltage and current supply lines. This voltage control circuit comprises:
means for producing a first trigger voltage in response to the voltage across the first and second lines, this first trigger voltage having an amplitude representative of the amplitude of the voltage across the first and second lines;
first switch means, connected in series with a high impedance element between the control terminal and one of the first and second lines, for establishing a high impedance current path between the control terminal and said one line when the first trigger voltage reaches a given amplitude, wherein the first switch means comprises means for producing a second trigger voltage having a first amplitude when the high impedance current path is not established and a second amplitude when the high impedance current path is established; and
second switch means, connected in series with a low impedance element between the control terminal and said one line, for establishing a low impedance current path between the control terminal and said one line when the second trigger voltage has the first amplitude.
Accordingly, when the first trigger voltage has an amplitude lower than the given amplitude, the high impedance current path is not established, a second trigger voltage of first amplitude is produced, and the low impedance current path is established to result in a voltage signal amplitude on the control terminal which disables the power controller unit and, when the amplitude of the first trigger voltage reaches the given amplitude, the high impedance current path is established, a second trigger voltage of second amplitude is produced, and the low impedance current path is not established to result in a voltage signal amplitude on the control terminal which enables said power controller unit.
The present invention is further concerned with a voltage control circuit for controlling the amplitude of a voltage signal on a control terminal of a power controller unit itself controlling a voltage and current supply source which supplies a current to a light-emitting load through first and second voltage and current supply lines. The voltage control circuit comprises:
a voltage divider circuit connected between the first and second lines and comprising resistors which divide the voltage on the first and second lines to produce a first trigger voltage signal;
a first controllable switch member connected in series with a high impedance element between the control terminal and one of the first and second lines to define a high impedance current path between this control terminal and said one line, this first controllable switch member being responsive to the first trigger voltage signal and having a first current-conductive junction established when the first trigger voltage reaches a given amplitude, wherein the high impedance current path produces a second trigger voltage having a first amplitude when the first current-conductive junction is not established and a second amplitude when the first current-conductive junction is established; and
a second controllable switch member connected in series with a low impedance element between the control terminal and said one line to define a low impedance current path between this control terminal and said one line, this second controllable switch member being responsive to the second trigger voltage and having a second current-conductive junction established when the second trigger voltage has the first amplitude and non established when the second trigger voltage signal has the second amplitude.
Therefore, when the first trigger voltage has an amplitude lower than the given amplitude, the first current-conductive junction is not established to produce in the high impedance current path a second trigger voltage of first amplitude which establishes both the second current-conductive junction and the low impedance current path to result in a voltage signal amplitude on the control terminal which disables the power controller unit and, when the amplitude of the first trigger voltage reaches the given amplitude, both the first current-conductive junction and the high impedance current path are established to produce in the high impedance current path a second trigger voltage of second amplitude whereby both the second current-conductive junction and the low impedance current path are not established to result in a voltage signal amplitude on the control terminal which enables the power controller unit.
The present invention is still further concerned with a voltage and current supply source responsive to alternating voltage and current from an ac source for supplying dc voltage and current to a light-emitting load, comprising:
a rectifier unit rectifying the alternating voltage and current from the ac source and supplying the rectified voltage and current to first and second voltage and current supply lines;
a converter of the rectified voltage and current into the dc voltage and current supplied to the light-emitting load;
a power controller unit having a control terminal and controlling the converter in response to the rectified voltage on the first and second lines; and
a voltage control circuit as described above, for controlling the amplitude of a voltage signal on the control terminal of the power controller unit.
The embodiments described herein present the advantage that they permit the use of LED lamps in applications, such as railway signal light applications, where there is a need for remote monitoring of the lamps, while keeping the advantageous features of lower power consumption and longer life.
Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings.